Mercy by Ren Loomis

When Mercy was six her parents disappeared and her grandparents moved into her home to raise her. Four years later Mercy disappeared, and two weeks after her disappearance her grandparents die of heart failure. Now, twelve years later, Jack and Mia St. Johns think they have found the perfect location for their annual Halloween party. But when they recruit some friends and the owner of the house, James Morrice, to help with party preparations they find out it may be just a little too perfect.